1. Field of the Invention
The present invention generally relates to a vehicle steering system. The present invention more specifically relates a vehicle steering system having a mechanical steering system and a steer-by-wire steering system where the steer-by-wire steering system is connected in normal operating situations and selectively disconnected in defective situations.
2. Background Information
Recently, a vehicle steering system has been developed which is capable of varying a steering characteristic by varying the ratio between an amount of operation of an operating member (steering wheel) and an amount of turning of the vehicle steered wheels. Thus, movement of the operating member operates a steering actuator to transmit a steering force to the vehicle steered wheels in order to vary the turning angle. One example of this type of vehicle steering system is known as a so called steer-by-wire system. In a so called steer-by-wire system, the operating member (steering wheel) is not mechanically connected to the vehicle steered wheels normal operating situations. In a steer-by-wire system, the ratio between the amount of operation and the amount of steering is varied by controlling the steering actuator, without mechanically connecting the steering wheel to the vehicle steered wheels, when movement of a steering actuator is transmitted to the vehicle steered wheels in order to vary the steering angle through a steering gear unit actuator. For example, in a steer-by-wire type steering system, since the steering wheel is not normally connected mechanically to the vehicle steered wheels, a reaction force is applied to the steering wheel based on friction with the road surface such as steering resistance to provide the driver with a steering feeling similar to a mechanically connected steering system.
Some of the steer-by-wire type steering systems for vehicles that can freely set a reaction torque for a steering wheel and a turning angle for the vehicle steered wheels, it is known to provided a mechanical back-up mechanism, which uses a clutch to connect and disconnect the steering wheel to and from the vehicle steered wheels to maintain safety in case when there is a system defect occurring. One example of such a steer-by-wire type steering system is disclosed in U.S. Pat. No. 6,442,462 (also see, Japanese Laid-Open Patent Publication No. 2002-145098). In U.S. Pat. No. 6,442,462,the steering wheel and the vehicle steered wheels are disconnected in normal situations, and a steering reaction force actuator and a steering gear unit actuator are operated to provide a steering reaction force to the steering wheel and a turning force to the vehicle steered wheels. When one of these force actuators malfunctions, a clutch is brought into engagement to mechanically connect the steering wheel to the vehicle steered wheels and apply a steering assist force.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved vehicle steering system. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.